Internet protocol (IP) multimedia subsystem (IMS) is a new architecture developed so that IP networks can rapidly deploy new IP-based multimedia services. An evolved packet system (EPS) is a next generation packet network architecture developed by the 3rd Generation Partnership Project (3GPP). EPS includes long term evolution (LTE) access networks and evolved packet core (EPC) networks (also referred to as system architecture evolution (SAE) networks). IMS over SAE/EPC networks is envisioned to be a next generation network architecture that supports a variety of access technologies and provides advanced IP multimedia services with seamless mobility.
To support emergency services (e.g., “911” calls) in IMS, a special serving call session control function (CSCF) has been developed for the IMS architecture, and is called an emergency serving CSCF (E-CSCF). The E-CSCF performs call processing of emergency calls. For example, the E-CSCF queries a location resource function (LRF) to determine a location of a caller, queries a route determination function (RDF) to determine where to route the emergency call, and actually routes the emergency call to a correct public safety answering point (PSAP) (e.g., a final destination of an emergency call where a “911” operator is located). In current “legacy” single-carrier radio transmission technology (1xRTT) networks, each mobile switching center (MSC) queries a mobile position center (MPC) to receive an emergency service routing key (ESRK) using a request message (e.g., an ORREQ message). Based on the returned ESRK, a serving MSC routes the emergency call to a selective router (SR) via a direct or dedicated trunk. The SR forwards the emergency call to a PSAP identified by the ESRK. Direct trunks (e.g., centralized automatic message accounting (CAMA) trunks, feature group D (FG-D) trunks, signaling system 7 (SS7) trunks, etc.) are required and used to connect a MSC and a SR. One of the main reasons for using direct or dedicated trunks is to shield the emergency network from outside interference. Once the emergency call reaches the SR, the SR routes the emergency call to a correct PSAP.
To support emergency calls in IMS-based networks, it has been proposed to route the IMS-based emergency calls to an emergency service IP PSAP network (i.e., an ESInet—a proposed emergency network that interconnects IP-capable PSAPs). Under such a proposal, IMS-based networks would deliver the emergency call (e.g., via IP) to the ESInet, and the ESInet would route the emergency call (e.g., via IP) to a PSAP. Eventually, the use of IP for emergency call delivery will preclude the use of direct trunks. However, it will still take significant time before all PSAPs are compatible with IP. Before the ESInet becomes available, IMS-based emergency calls need to be routed to several hundred legacy SRs which require direct trunks. Since IMS-based networks are typically deployed at a few centralized locations via a small number of media gateways that reach legacy (e.g., the Public Switched Telephone Network (PSTN)) networks, supporting emergency call routing to several hundred SRs would require setting up thousands direct trunks (e.g., including redundant connections) to the several hundred SRs from a few IMS media gateway locations. Implementing thousands of direct trunks to support IMS-based emergency calls would be expensive to setup and maintain.